Canada has some of the largest deposits of a heavy oil called bitumen. Unfortunately, the bitumen is especially difficult to recover because it is wrapped around sand and clay, forming what is call ‘oil sands.’ Bitumen is a thick, sticky form of crude oil, so heavy and viscous (thick) that it will not flow unless heated or diluted with lighter hydrocarbons. Indeed, the crude bitumen contained in the Canadian oil sands is described as existing in the semi-solid or solid phase in its natural deposits.
Conventional approaches to recovering heavy oils such as bitumen often focus on lowering the viscosity through the addition of heat. Commonly used in situ extraction thermal recovery techniques include a number of reservoir heating methods, such as wellbore heating, wellbore combustion, hot fluid injection, steam flooding, cyclic steam stimulation, Steam Assisted Gravity Drainage (SAGD), in situ combustion, and variations thereon.
SAGD is the most extensively used technique for in situ recovery of bitumen resources. In SAGD, steam is injected continuously into the injection well, where it rises in the reservoir and forms a steam chamber. The heat from the steam reduces the hydrocarbon's viscosity, thus enabling the heated crude and condensed steam to flow down to the production well and be transported to the surface via pumps or lift gas. The produced hydrocarbon thus is a mixture of hydrocarbon and water. SAGD is very water intensive, requiring 3-5 barrels of water to produce a barrel of oil. It is also very energy intensive, as considerable energy is needed to generate the steam.
One improvement to SAGD that has the potential to reduce water and energy usage is combining solvent injections with SAGD. For instance, expanding solvent SAGD (ES-SAGD) injects a low concentration hydrocarbon additive with the steam. The additive condenses with the steam at the boundary of the steam chamber causing oil dilution and further viscosity reduction. In solvent-cyclic-SAGD (SC-SAGD), the wells are started with steam and quickly progress to the addition of solvents. The initial solvent is often a heavy solvent, with lighter solvents being injected over time. The amount of steam injected declines as the solvent is injected.
Even though, the industry is moving toward injection of solvent in the reservoir to lower steam-to-oil ratios, the purchase/transportation of make-up solvent (beyond typically recovered solvent) can be expensive and every effort to reduce solvent usage or increase its recovery for re-use contributes to efficiency and cost effectiveness.
As mentioned, SAGD requires large amounts of water in order to generate a barrel of oil. Because water is as precious a resource as oil, the “produced water” is usually recycled. It is thus cleaned and returned to the boiler, where it is converted into steam and re-injected back into the ground.
Due to the recycling of water in SAGD operations, and the fact that the water encounters petroleum deposits as well as any additives used in production, the feedwater used to make steam is typically far from pure. Produced water and brackish well water are the main boiler feedwater sources and the contaminants in these two water sources differ. Water separated from the produced oil emulsion (produced water) is high in silica and in soluble organic compounds (see e.g., Table 1). Brackish well water, in contrast, can be high in hardness ions (calcium and magnesium) (see e.g., Table 2). The combination of these waters can be unstable and can produce a variety of mineral scales.
TABLE 1Range of typical solute concentrations in produced feedwaterComponentMinimumMaximumCa (mg/l)152Mg (mg/l)1.614K (mg/l)14240Na (mg/l)1303000SiO2 (mg/l)11260TOC (mg/l)170430NH3 (mg/l)1164Cl (mg/l)484800pH (s.u.)7.38.8“M” alkalinity (mg/l as CaCO3)1401400
TABLE 2Range of typical solute concentrations in brackish wellwaterComponentMinimumMaximumCa (mg/l)2.045Mg (mg/l)1.532K (mg/l)2.2250Na (mg/l)7003700SiO2 (mg/l)810TOC (mg/l)nd5NH3 (mg/l)nmnmCl (mg/l)4805300“M” (mg/l as CaCO3)8801200nd = not detectednm = not measured
Traditional boilers typically cannot accommodate these impurities, and have a great tendency to foul, thus increasing down time and contributing to costs. Water treatment equipment for removing organic and inorganic constituents, however, only adds to the capital and operating costs in preparing water for traditional boilers. Therefore, any technology that can reduce water or steam consumption has the potential to have significant positive environmental and cost impacts.
Thus, further improvements to steam generation methods are desired to improve recovery and cost efficiency in SAGD, ES-SAGD and other steam based enhanced recovery methods. In particular, a method that reduces water usage, reduces fouling, and at the same time reduces solvent costs would be greatly beneficial.